The present invention relates to methods and devices used to measure and record physiological data, such as blood pressure and electrocardiogram data. More particularly, the invention relates to a method and a system of distinguishing pressure pulses to avoid potential harm to patients while making physiological measurements.
Modem medical practice involves monitoring a variety of physiological data, including electrical activity and blood pressure. Electrocardiograms (xe2x80x9cECGxe2x80x9d) are used to measure electrical activity that controls the contraction of the heart. As is known, prominent parts of an ECG are the P wave, a deflection caused by the current originating in the atrium; the QRS complex, which represents the electrical activity of the ventricles as they contract; and the T wave, which denotes ventricular relaxation. These changes in electrical activity may, in general, be sensed using electrodes attached to the body.
The pulsatile pressure of blood in a large or xe2x80x9cgreatxe2x80x9d artery of the body (the xe2x80x9carterial pressure waveformxe2x80x9d) may be sensed by introducing a fluid-filled catheter into a major artery of the systemic circulation, such as the radial, brachial, or femoral artery, and connecting it to a pressure transducer. The arterial pressure waveform represents the mechanical activity of the heart, and therefore follows, or lags, the electrical activity. The upstroke of the pressure waveform follows the QRS complex of the ECG and is due to the contraction of the left ventricle as blood is forced into the aorta through the aortic valve. The peak of the pressure pulse is the peak systolic pressure. A small dip, called the dicrotic notch, caused by the closure of the aortic valve, may be observed on the downstroke of the pulse. The gradual decrease in pressure after the dicrotic notch is due to run-off of the blood to the peripheral arteries.
Blood pressure may also be monitored in other parts of the body. The pressure may be measured in the pulmonary artery though the use of a pulmonary artery catheter. In this case, the pulmonary artery catheter is introduced into the body through a major vein such as the femoral or subdlavian vein. It is threaded through the vena cava, the right atrium, the right ventricle, and rests in a branch of the pulmonary artery. At the distal tip of the pulmonary artery catheter is an opening to a fluid-filled lumen through which the pulmonary artery pressure is sensed. The appearance of the pulmonary artery pressure waveform is similar to the arterial pressure waveform described above, although the pressures are about one sixth the value of the pressures in the major arteries. The pressure pulse is created as blood flows through the pulmonary valve due to right ventricular contraction followed by run-off into the pulmonary circulation. Pulmonary artery catheterization is a very invasive procedure with many associated risks. The procedure is generally used in circumstances where a patient has a severe medical condition that requires intensive care and observation.
One measurement that may be made with the pulmonary artery catheter in place is the pulmonary artery wedge pressure (xe2x80x9cPAWPxe2x80x9d). In this measurement, a balloon near the end of the catheter is inflated, occluding flow through that branch of the pulmonary circulation. (The balloon is said to be xe2x80x9cwedgedxe2x80x9d in the pulmonary artery.) The stagnant blood at the distal tip of the catheter is, in effect, at the same pressure as the blood in the left atrium, which, when the mitral valve is open, is also at the same pressure as the blood in the left ventricle. One hazard associated with the pulmonary artery wedge pressure measurement is that the catheter balloon may be over-inflated by the clinician, resulting in a tear or rupture of the pulmonary artery. As should be apparent, a rupture of the pulmonary artery can have dire consequences for the patient.
During the PAWP measurement, the pulmonary artery waveform takes on a dampened appearance, as the pressure waveform represents the left atrial pressure rather than the pulsatile pulmonary artery pressure. Prominent parts of the PAWP waveform may be identified and correlated to the ECG. An a wave is produced by left atrial contraction and follows the P wave of the ECG. The descending portion of the a wave is called the x-descent, reflecting left atrial relaxation. A small positive deflection is sometimes visible on the x-descent. This deflection, called the c wave, is produced by the closure of the mitral valve. The v wave is produced by the filling of the left atrium against the closed mitral valve during ventricular systole and, therefore, occurs after the R wave of the ECG (more precisely, it occurs after the T wave of the ECG). The downstroke following the peak of the v wave is termed the y-descent, which represents the opening of the mitral valve and a decrease in left atrial pressure and volume during passive emptying into the left ventricle.
The v wave of the PAWP waveform is exaggerated and elevated in patients with mitral insufficiency as the mitral valve does not completely close during the ventricular contraction, causing a regurgitation of blood back into the left atrium. This condition is commonly referred to as mitral valve regurgitation (MVR). PAWP measurements can become more difficult to take in this case because the large v waves may resemble the unwedged pulmonary artery pressure pulse and lead the unwary clinician to believe that the catheter has not properly wedged in the pulmonary artery. This can result in repeated attempts to wedge the catheter (with the risk of PA rupture or perforation). Conversely, it may result in prolonged wedging if the clinician erroneously believes the catheter is in the unwedged state and goes about his duties (with the risk of pulmonary infarction).
It is believed that a major cause of balloon over-inflation and subsequent pulmonary artery rupture during PAWP measurements is a failure by the clinician to recognize and differentiate between large v waves and pulmonary artery pressure pulses on pressure tracings, which are typically presented electronically on a monitor or similar display.
Accordingly, it would be desirable to have a method and system for differentiating between pressure pulses due to right ventricular contraction and v waves during the PAWP measurement. The invention provides a method and a system for differentiating between regular pulmonary artery pressure pulses and v waves sensed during a PAWP measurement. The method involves computing or determining the timing or the phase delay between the peak of a PA pressure pulse and the peak of the corresponding arterial pressure pulse. The method also involves observing a change in the phase or timing between the arterial pressure and the PA pressure pulses.
Typically, the arterial and PA pressure pulses occur almost simultaneously. However, v waves occur later in the cardiac cycle than the regular pressure pulse peak. The inventors have observed that a change of timing or phase delay between the arterial pulse peak and the observed peak of the PA waveform indicates that a wedge is in place and that large v waves have been detected. Thus, the invention further includes an indication of a phase change between the arterial and PA pressure peaks, such as a plot of the phase difference over time. The inventors have observed that such as plot yields readily identifiable balloon inflation and deflation times in the form of step responses.
The method involves using a reference signal for timing. In one embodiment of the invention, the PA peak pressure time may be measured with respect to the R wave of an ECG waveform. In another embodiment of the invention, the PA peak pressure time is measured with respect to the peak pressure of an arterial pressure signal. Other reference waveforms, including plethysmograms, may also be used. Preferably, a combination of ECG and arterial pressure timings are used.
The peak PA pressure is monitored and stored on a beat-by-beat basis, and the phase measurement is calculated in a processor of a monitoring system. The monitor alerts the clinician that a wedge is in place and that further inflation should not occur. In addition to notifying the clinician of a wedged balloon during intentional PAWP measurements, the invention may also be used to alert a clinician that a spontaneous (unintentional) wedge has occurred during normal PA monitoring.
As is apparent from the above, it is an advantage of the present invention to provide a method and system of differentiating between regular pressure pulses and v waves measured during patient monitoring. Other features and advantages of the present invention will become apparent by consideration of the detailed description and accompanying drawings.